p-phenylenediamine is a compound of various applications such as cosmetic, antioxidant, fuel additive and dyestuff and, more particularly, a useful compound as a base material of a highly resilient and heat-resistant material (e.g., aramide fiber) and a raw material of polyurethane, p-phenylenediisocyanate.
Conventionally, there have been used two commercialized methods for preparing p-phenylenediamine of such a wide use range. The first method involves reacting chlorobenzene with nitric acid and reacting the resulting product, p-nitrochlorobenzene with ammonia to yield 4-nitroaniline, followed by hydrogenation of 4-nitroaniline to produce p-phenylenediamine. The second method involves diazotation of aniline using nitrogen oxide, reaction with an excess of aniline to form 1,3-diphenyltriazene, and rearragement of 1,3-diphenyltriazene to 4-aminoazobenzene and hydrogenation of 4-aminoazobenzene to yield p-phenylenediamine and aniline.
The first method is most widely used and yields isomers of nitrochlorobenzene, i.e., ortho- and para-forms at a ratio of 65:35 through nitration of chlorobenzene in order to form an intermediate, 4-nitroaniline. At this stage, however, the final product is not isolated as pure p-phenylenediamine but stained with a small amount of isomers. Thus this method is normally used to prepare p-phenylenediamine of low purity or dyes. Another disadvantage of the method is to dispose of a large quantity of wastewater due to the chlorine-containing materials used.
The second method solves the problem related to production of an isomer such as ortho-form. Specifically, the method involves diazotating aniline with nitrogen oxide and reacting the resulting diazo-compound with an excess of aniline to yield 1,3-diphenyltriazene, which is then subjected to rearrangement and hydrogenation reaction to provide aniline such as p-phenylenediamine (See. U.S. Pat. Nos. 4,020,052 and 4,279,815). This method may produce a relatively pure p-phenylenediamine but requires complicated multi-stage reactions.
Recently, a method of preparing 4-nitroaniline in two steps has been developed (See. J. Am. Chem. Soc., 1992, 114(23); J. Org. Chem., 1993, 58, 6883-6888; U.S. Pat. Nos. 5,436,371 and 5,380,946; and WO 93/24447). The method includes the steps of performing the NASH (Nucleophilic Aromatic Substitution of Hydrogen) to synthesize an intermediate, N-(4-nitrophenyl)benzamide through the reaction of benzamide and nitrobenzene with an organic base, and adding water (or ammonia) to the intermediate to decompose the intermediate into 4-nitroaniline and benzoic acid (or benzamide). This method provides 4-nitroaniline in the para-form with a relatively high selectivity and may be used to prepare highly pure p-phenylenediamine through hydrogenation. However, the method produces 4-nitroaniline in two stages with a low yield due to the water content in the reaction mixture. Furthermore, it requires a need of using a relatively high-cost material, TMA(OH) as an organic base.